Why is NaN === NaN false? [duplicate]

Question

Why does NaN === NaN return false in Javascript?

> undefined === undefined true > NaN === NaN false > a = NaN NaN > a === a false 

On the documentation page I see this:

Testing against NaN

Equality operator (== and ===) cannot be used to test a value against NaN. Use isNaN instead.

Is there any reference that answers to the question? It would be welcome.

Answer 1

Strict answer: Because the JS spec says so:

• If Type(x) is Number, then
  • If x is NaN, return false.
  • If y is NaN, return false.

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Useful answer: The IEEE 754 spec for floating-point numbers (which is used by all languages for floating-point) says that NaNs are never equal.